The present invention relates to addition polymers, in particular to methods of manufacturing such polymers by dispersion polymerisation techniques.
Acrylic polymers, such as polymethyl methacrylate are well known and widely used commercially important examples of addition polymers. They may be prepared by various methods, including by bulk polymerisation, emulsion polymerisation or polymerisation in solution. Dispersion polymerisation has the advantage of keeping the viscosity of the polymerisation mixture low and enabling the morphology of the polymer to be controlled and so is particularly suitable for producing high molecular weight polymer beads.
Polymerisation in supercritical fluids, especially supercritical carbon dioxide (sCO2) has been demonstrated to have the advantages of allowing good control of particle size whilst also producing polymer with a low residual monomer concentration. In addition, the CO2 does not contaminate the resulting polymer. EP-A0735051 describes the free-radical polymerisation of styrene which includes heating a monomer, initiator and a free-radical agent in supercritical CO2.
WO-A-9504085 describes the use of high molecular weight fluorinated graft copolymers and block copolymers for use as surfactants in emulsion polymerisations using supercritical CO2 as the continuous phase. The emulsion polymerisation of acrylamide in a sCO2 continuous phase with an amide-functionalised fluoropolymer used as an emulsifier is described by Beckman et al in Macromolecules 1994 p 312.
In dispersion polymerisation the monomer is dissolved in the reaction medium and the resulting polymer, which is not soluble in the reaction medium, must be kept dispersed to enable the polymerisation reaction to be carried out efficiently and to control the resulting polymer particle morphology. A stabiliser may be added to the reaction mixture to keep the polymer produced in the reaction in dispersion. The requirement of a stabiliser compound for this purpose is that it is soluble in the reaction medium, e.g. the supercritical fluid and has an affinity for the polymer. Existing technology uses high molecular weight block or graft copolymers which can effectively wrap and coat the growing polymer particle, thus maintaining a stable dispersion and facilitating control of the reaction.
Dispersion polymerisation in s-CO2 using block copolymers is described in ACS Polymer Preprints 1997, p400; Macromolecules 1995 28 p.8159 (DeSimone et al). The use of graft copolymers as dispersion polymerisation stabilisers is described in Macromolecules 1997 30 p. 745 (Beckman et al) in which the stabiliser described has perfluoropolyether chains grafted onto an acrylic backbone. DeSimone describes the use of a methyl methacrylate-terminated polydimethyl siloxane polymer as a stabiliser for the dispersion polymerisation of methyl methacrylate (Macromolecules 1996 29 p.2704). This stabiliser has a polymerisable end-group and thus remains bound into the structure of the resulting polymer.
The stabiliser materials described in the prior art are usually required to be used at high concentration (typically 1-2% or more w/w based on monomer) and, since they are relatively complex molecules to prepare, they are relatively expensive materials to use. Another problem associated with the use of these stabilisers is that they tend to remain in the finished polymer and can be difficult to remove completely. Also they may not be completely recoverable from the reaction, which further adds to the expense of using them.
It is therefore an object of the present invention to provide a method of producing an acrylic polymer by dispersion polymerisation in supercritical fluids which overcomes some of the above-mentioned problems.
According to the invention a method of producing a polymer comprises the steps of forming a homogeneous reaction mixture comprising at least one addition polymerisable monomer, a fluid reaction medium, and a stabiliser, wherein the stabiliser comprises a chain which is soluble in the fluid and a functional end-group which is not polymerisable by a free-radical mechanism and polymerising said at least one monomer in the reaction mixture.
According to a second aspect of the invention, a stabiliser for use in dispersion polymerisation of acrylic monomers in a fluid reaction medium comprises a material having a chain which is soluble in the fluid reaction medium and a functional end-group which is not polymerisable by a free-radical mechanism.
The monomer may be any suitable unsaturated compound which is useful in the formation of addition polymers. Suitable monomers include, but are not limited to, optionally functionalised or substituted vinyl monomers such as styrene, acrylic monomers, vinyl chloride, vinyl acetate substituted olefins and maleic anhydride. A preferred group of monomers comprises esters of acrylic or methacrylic acids, their alkyl esters, and substituted analogues thereof. More than one monomer may be present if a copolymer product is required. Preferred monomers include alkyl acrylates, methacrylates and ethacrylates, especially methyl (meth)acrylate, ethyl (meth)acrylate and butyl (meth)acrylate.
The reaction is preferably carried out in the presence of a free-radical initiator. Suitable initiators include azo-compounds such as azobis(isobutyronitrile) (AIBN), azobis(4-methoxy-2,4-dimethylvaleronitrile (commercially available as xe2x80x9cV-70xe2x80x9d), and peroxides such as dicumyl peroxide and t-butyl peroxide.
The fluid reaction medium may comprise any known fluid in which the monomer is soluble and preferably comprises a fluid which may be brought into supercritical state as commonly known in the art. As is known in the art such fluids may be subjected to conditions of temperature and pressure up to a critical point at which the equilibrium line between liquid and vapour regions disappears. Supercritical fluids are characterised by properties which are both gas-like and liquid-like. In particular the fluid density and solubility properties resemble those of liquids, whilst the viscosity, surface tension and fluid diffusion rate in any medium resemble those of a gas, giving gas-like penetration of the medium.
Preferred fluids include carbon dioxide, di-nitrogen oxide, carbon disulphide, aliphatic C2-10 hydrocarbons such as ethane, propane, butane, pentane, hexane, ethylene, and halogenated derivatives thereof such as for example carbon hydrogen trifluoride or chloride and HCF134a, C6-10 aromatics such as benzene, toluene and xylene, C1-3 alcohols such as methanol and ethanol, sulphur halides such as sulphur hexafluoride, ammonia, xenon, krypton and the like. Typically these fluids may be brought into supercritical conditions at temperature of between 0-150xc2x0 C. and pressures of 7-1000 bar, preferably 12-800 bar. It will be appreciated that the choice of fluid may be made according to its properties, for example diffusion and solvent properties. The choice of fluid may also be made with regard to critical conditions which facilitate the commercial preparation of the polymer.
The preferred fluid comprises supercritical carbon dioxide, optionally in admixture with a further fluid. The advantages of using carbon dioxide include the fact that it forms a supercritical fluid at relatively low temperatures (32xc2x0 C. at 74 bar), is readily available and easy to handle and can be removed from the reaction mixture by venting, leaving little residue.
The temperatures and pressures used depend upon the nature of the fluid used and the conditions under which it exhibits supercritical properties. The reaction need not be carried out under supercritical conditions and the fluid may be a liquid when the temperature is below the supercritical range of the fluid. Also the temperature and pressure at which the reaction is carried out is dependent upon the nature of the initiator used, as is known in the art. In a preferred system, for the polymerisation of acrylic materials in supercritical carbon dioxide, the reaction is preferably carried out at pressures in the range 1,000-10,000 psi, more preferably 1,500-7,000 psi at temperatures of between about 0-150xc2x0 C., preferably between about 40-80xc2x0 C., e.g. about 70xc2x0 C. when the initiator used is AIBN.
The stabiliser comprises a chain which is soluble in the fluid and a functional end-group. The fluid-soluble chain may comprise fluoropolymers, siloxanes, polyphosphazenes, polyethylene oxides or other polymer chains which are soluble in the chosen supercritical fluid. Preferably the stabiliser comprises a functionalised fluoropolymer, especially a functionalised perfluoro-polyether, which is preferably terminally functional. Suitable end-groups comprise a carboxylic acid, amide ester, amine, acid chloride, alcohol, phosphate or like group. Preferably the stabiliser comprises a carboxylic acid end-group. A particularly preferred stabiliser comprises a carboxylic acid terminated perfluoro polyether. The stabiliser may be monofunctional or polyfunctional, e.g. difunctional, however monofunctional stabilisers are preferred. The molecular weight of the stabiliser may vary widely, e.g. between about 300 and 106 Daltons (D). We have found that materials having a molecular weight (Mw) in the range 1000-10,000 are particularly effective as stabilisers in some preferred systems. We have found that suitable such stabilisers include carboxylic acid terminated perfluoro polyether materials such as those sold under the trade names KRYTOX(trademark) 157 FSL, 157 FSM, 157 FSH by DuPont, GALDEN(trademark) MF300 or FOMBLIN(trademark) DA601 as sold by Ausimont. We have found that the use of such materials as a stabiliser allows good control of the morphology of the polymer particles produced and effectively stabilises the polymerisation. A further advantage offered by the use of these materials is that the stabiliser appears not to become incorporated into the polymer and may be removed from the polymer relatively easily by venting along with the fluid solvent.
The concentration of stabiliser in the reaction mixture is preferably in the range 1xc3x9710xe2x88x925xe2x88x9240 wt % with respect to the monomer concentration, more preferably 0.01-10%. We have found that the concentration of the stabiliser affects the morphology of the polymer particles produced in the reaction. By varying the stabiliser concentration, the resulting polymer may have a morphology which vanes from isolated spherical particles of mean diameter 0.5-5 xcexcm to elongated chains of agglomerated particles which form open porous structures of high surface area. At low stabiliser concentrations, nodular morphologies may be formed. At a stabiliser concentration of 0.1-35% when the fluid used is supercritical CO2, the polymerisation of methyl methacrylate produces well dispersed particles and the stabiliser may be removed easily by venting. The morphology produced may also be controlled by controlling the density of the supercritical fluid.
The molecular weight of the polymer produced may vary widely e.g. from 20,000-400,000 Daltons (Mw). We have found that a particular advantage of the method of the invention using the stabilisers described above is that the yield of polymer produced may be relatively high. For example, typical yields achievable are at least 85% when the polymer molecular weight is in the range 130,000-300,000.
The polymerisation mixture may include other additives, such as chain transfer agents for example. Chain transfer agents are commonly used to produce polymer which is more thermally stable than normal radical-terminated polymer. Suitable chain transfer materials are well known and include a range of mercaptans. A further advantage of the polymerisation method of the invention is that residual chain transfer agent may be easily removed from the polymer by venting with the fluid medium.